1. Field of the Invention
The invention relates to a servo-assisted steering system for motor vehicles The servo-assisted steering system includes a steering valve that has two intake seat valves and two outlet seat valves, which can be adjusted via an actuating device as a function of a rotary motion of a steering shaft. The closure bodies of the outlet seat valves can be acted upon via their actuating tappets by the force of centering springs in the direction toward the actuating device, so that the actuating tappets contact the actuating device. The closure bodies of the intake seat valves can be acted upon in the closing direction by the force of compression springs, so that the intake seat valves are closed in the neutral position of the steering valve. The outlet seat valves are open in the neutral position of the seat valve. The intake seat valves are not opened until after the closure of the respective associated outlet seat valves.
2. Description of the Prior Art
This kind of servo-assisted steering system with a closed center is known from German Patent Disclosure DE-A1 41 26 020. Such a servo-assisted steering system has a linear characteristic curve course. This means that at high servo-pressures, the actuating moment can rise sharply.
In modern servo-assisted steering systems with an open center, however, it is usual for the actuating moment to be increased only insignificantly beyond a certain pressure in the servomotor. This means that beyond this certain point, the characteristic curve has a steeper course. This effect is known as "cutoff" or "actuating moment limitation".
The object of the invention is to improve a servo-assisted steering system of this generic type, that is, a servo-assisted steering system having a closed center, in such a way that with its steering valve, characteristic curves can be generated that in their form come as close as possible to the characteristic curves of conventional modern servo-assisted steering systems with an open center.